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6.4 LEARNING FROM THE RECOVERED STEELEdit
6.4.1 Collection of Recovered SteelEdit
NIST had two reasons for obtaining specimens of structural steel from the collapsed towers. The primary objective was characterizing the quality of the steel and determining its properties for use in the structural modeling and analysis of the collapse sequences. The second reason was obtaining information regarding the behavior of the steel in the aircraft impact zone and in areas which had major fires.
Table 6–2. Indications of major structural changes up to collapse initiation. Tower Time (a.m.) Observation 10:18 Smoke suddenly expelled on the north face (floors 92, 94, 95 to 98) and west face (92, 94 to 98). 10:23 Inward bowing of perimeter columns on the east side of the south face from floors 94 to 100; maximum extent: 55 in. ± 6 in. at floor 97. WTC 1 10:28:22 First exterior sign of collapse (downward movement of building exterior). Tilting of the building section above the impact and fire area to due south as the structural collapse initiated. First exterior sign of downward movement of building at floor 98. 9:02:59 Exterior fireball from the east face of floor 82 and from the north face from floors 79 to 82. The deflagration prior to the fireballs may have caused a significant pressure pulse to act on floors above and below. 9:21 Inward bowing of exterior wall columns on most of the east face from floors 78 to 83; maximum extent: 7 in. to 9 in. at floor 80. WTC 2 9:58:59 First exterior sign of collapse (downward movement of building exterior). The northeast corner tilted counterclockwise around the base of floor 82. Column buckling was then seen progressing across the north face and nearly simultaneously on the east face.
Tilting of the building section above the impact and fire area to the east and south prior to significant downward movement of the upper building section. The tilt to the south did not increase any further as the upper building section began to fall, but the tilt to the east did increase until dust clouds obscured the view.
Within weeks of the destruction of the WTC, contractors of New York City had begun cutting up and removing the debris from the site. Members of the FEMA-sponsored and ASCE-led [[Building Performance Assessment Team]], members of the Structural Engineers Association of New York, and Professor A. Astaneh-Asl of the University of California, Berkeley, CA, with support from the [http://en.wikipedia.org/wiki/ National Science Foundation ], had begun work to identify and collect WTC structural steel from various recycling yards where the steel was taken during the clean-up effort. The [[Port Authority of New York and New Jersey]] (Port Authority) also collected structural steel elements for future exhibits and memorials. Over a period of about 18 months, 236 pieces of steel were shipped to the NIST campus, starting about six months before NIST launched its Investigation. These samples ranged in size and complexity from a nearly complete three-column, three-floor perimeter assembly to bolts and small fragments. Figures 6–3 through 6–5 show some of the recovered steel pieces. Seven of the pieces were from WTC 5. The remaining 229 samples represented roughly 0.25 percent to 0.5 percent of the 200,000 tons of structural steel used in the construction of the two towers.
The collection at NIST included samples of all the steel strength levels specified for the construction of the towers. The locations of all structural steel pieces in WTC 1 and WTC 2 were uniquely identified by stampings (recessed letters and numbers) and/or painted stencils. NIST was successful in finding and deciphering these identification markings on many of the perimeter panel sections and core columns, in many cases using metallurgical characterization to complete missing identifiers. In all, 42 exterior panels were positively identified: 26 from WTC 1 and 16 from WTC 2. Twelve core columns were positively identified: eight from WTC 1 and four from WTC 2. Twenty-three pieces were identified as being parts of trusses, although it was not possible to identify their locations within the buildings.
Source: NIST. Figure 6–3. Examples of a WTC 1 core column (left) and truss material (right). Source: NIST. Figure 6–4. WTC 1 exterior panel hit by the fuselage of the aircraft.
Overlaying the locations of the specimens with photographs of the building exteriors following the aircraft impact (for perimeter columns and spandrels) and the extent-of-damage estimates (Section 6.8) (for core columns) enabled the identification of steel pieces near the impact zones. These included five specimens of exterior panels from WTC 1 and two specimens of core columns from each of the towers.
6.4.2 Mechanical and Physical PropertiesEdit
NIST determined the properties of many of the recovered pieces for comparison with the original purchase requirements, comparison with the quality of steel from the WTC construction era, and input to the structural models used in the Investigation. Structural steel literature and producers’ documents were used to establish a statistical basis for the variability expected in steel properties. Reconstruction of the Collapses
The properties of the steel samples tested were consistent with the specifications called for in the steel contracts. In particular, the yield strengths of all samples of the floor trusses were higher than called for in the original specifications. This was in part because the truss steels were supplied as a higher grade than specified. Overall, approximately 87 percent of all perimeter and core column steel tested exceeded the required minimum yield strengths specified in design documents. Test data for the remaining samples were below specifications, but were within the expected variability and did not affect the safety of the towers on September 11, 2001. Furthermore, lower strength values measured by NIST could be expected due to (a) differences in test procedures from those used in the qualifying mill tests and (b) the damaged state of the samples. The values of other steel properties were similar to typical construction steels of the WTC construction era. The limited tests on bolts indicated that their strengths were greater than the specified minimum, and they were stronger than contemporaneous literature would suggest as typical. The tested welds performed as expected. NIST measured the stress-strain behavior at room temperature (for modeling baseline performance), high temperature strength (for modeling structural response to fire), and at high strain rates (for modeling the aircraft impact). Based on data from published sources, NIST estimated the thermal properties of the steels (specific heat, thermal conductivity, and coefficient of thermal expansion) and creep behavior for use in the structural modeling of the towers’ response to fire.
6.4.3 Damage AnalysisEdit
NIST performed extensive analyses of the recovered steel specimens to determine their damage characteristics, failure modes, and (for those near the fire zones) fire-related degradation. In some cases, assessment of enhanced photographic and video images of the towers enabled distinguishing between damage that occurred prior to the collapse and damage that occurred as a result of the collapse. Because the only visual evidence was from the outside of the buildings, this differentiation was only possible for the perimeter panel sections. The observations of fracture and failure behavior, confirmed by an Investigation contractor, were also used to guide the modeling of the towers’ performance during impact and subsequent fires and to evaluate the model output. Source: NIST. Figure 6–5. WTC 1 exterior panel hit by the nose of the aircraft. Chapter 6 90 NIST NCSTAR 1, WTC Investigation For two of the five exterior panels from the impact zone of WTC 1, the general shape and appearance of the recovered pieces matched photographs taken just before the building collapse. Thus, NIST was able to attribute the observed damage to the aircraft impact. NIST also made determinations regarding the connections between structural steel elements: • There was no evidence to indicate that the joining method, weld materials, or welding procedures were inadequate. Fractures of the columns in areas away from a welded joint were the result of stretching and thinning. Perimeter columns hit by the plane tended to fracture along heat-affected zones adjacent to welds. • The failure mode of spandrel connections varied. At or above the impact zone, bolt hole tearout was more common. Below the impact zone, it was more common for the spandrels to be ripped from the panels. There was no evidence that fire exposure changed these failure modes. • The exterior column splices at the mechanical floors, which were welded in addition to being bolted, generally did not fail. The column splices at the other floors generally failed by bolt fracture. • The perimeter truss connectors (or seats) below the impact zone in WTC 1 were predominantly bent down or torn off completely. Above the impact zone, the seats were as likely to be bent upward as downward. Core seats could not be categorized since their asbuilt locations could not be determined. • Failure of core columns was a result of both splice connection failures and fracture of the columns themselves.
Examination of photographs showed that 16 of the exterior panels recovered from WTC 1 were exposed to fire prior to the building collapse. None of the nine recovered panels from within the fire floors of WTC 2 were directly exposed to fire. NIST used two methods to estimate the maximum temperatures that the steel members had reached:
• Observations of paint cracking due to thermal expansion. Of the more than 170 areas examined on 16 perimeter column panels, only three columns had evidence that the steel reached temperatures above 250 °C: east face, floor 98, inner web; east face, floor 92, inner web; and north face, floor 98, floor truss connector. Only two core column specimens had sufficient paint remaining to make such an analysis, and their temperatures did not reach 250 °C. NIST did not generalize these results, since the examined columns represented only 3 percent of the perimeter columns and 1 percent of the core columns from the fire floors. • Observations of the microstructure of the steel. High temperature excursions, such as due to a fire, can alter the basic structure of the steel and its mechanical properties. Using metallographic analysis, NIST determined that there was no evidence that any of the samples had reached temperatures above 600 ºC.
These results were for a very small fraction of the steel in the impact and fire zones. Nonetheless, these analyses indicated some zones within WTC 1 where the computer simulations should not, and did not, predict highly elevated steel temperatures.